1. Field of the Invention
The invention relates to image sensor systems, and in particular, the present invention relates to an image sensor with improved color reproduction capability.
2. Description of the Related Art
Digital photography is one of the most exciting technologies that have emerged in the past years. With the appropriate hardware and software (and a little knowledge), anyone can put the principles of digital photography to work. Digital cameras, for example, are on the cutting edge of digital photography. Recent product introductions, technological advancements, and price cuts, along with the emergence of email and the World Wide Web, have helped make digital cameras the hottest new category of consumer electronics products.
Most digital cameras use an image sensor or a photosensitive device, such as a charged-coupled device (CCD) or a Complementary Metal-Oxide Semiconductor (CMOS) device to sense a scene. The photosensitive device reacts to light reflected from the scene and can translate the strength of that reaction into electronic charging signals that are further digitized. Because the image is actually a collection of numeric data, the image data can easily be downloaded into a computer and manipulated for more artistic effects.
FIG. 1 illustrates an image sensor or photosensitive chip 100 which may be used in an image capturing device, such as a digital camera, for either stationary or video photography. Image sensor 100 produces digital image data representing the light intensity of the scene being captured. Image sensor 100 is typically fabricated on a single substrate, such as in a CMOS fabrication process, and includes a two-dimensional array of photosensitive devices, also referred to as photodetectors. The number of the photodetectors in image sensor 100 typically determines the resolution of digital images resulting therefrom. The horizontal resolution is a function of the number of photodetectors in a row 102, and the vertical resolution is a function of the number of photodetectors in a column 104.
Each of the photodetectors in an image sensor comprises a photosensor that produces an electronic signal when it is exposed to light. Generally, the photosensor is a photodiode or a photogate in a CMOS sensor. FIG. 2 illustrates an exemplary structure of a photodetector where a photodiode 120 is modeled as a current source 122 and a capacitor 124. When a reset signal is applied at a Reset terminal 130, the capacitor 124 is fully charged to nearly the Vcc voltage through an NMOS transistor 128. After capacitor 124 is fully charged, photodiode 120 is ready for light integration.
When photodetector 120 is exposed to light, photons from incident light 126, filtered through a filter 136, impinge upon photodetector 120 and cause a change in the conductivity of the photodetector which is represented as a change in the current flow of current source 122. Current source 122 discharges capacitor 124 at a rate dependent on the number of photons striking photodetector 120. The voltage across capacitor 124 is thus dependent on the total number of photons striking photodetector 120. An output voltage signal Vout generated at output node 129, which is the voltage across capacitor 124, is indicative of the integrated light intensity between the time that transistor 128 is turned off and the time that light 126 incident on photodetector 120 is turned off or the time the readout process begins. An optional circuit 132 may be included to enhance the electronic signal Vout to a desired level so that the output signal, i.e. the pixel charge signal, can be effectively coupled to the subsequent circuitry in the image sensor. The pixel charge signal is subsequently digitized to provide pixel data for the associated pixel corresponding to the pixel charge signal.
Image sensor 100 can be used in black-and-white imaging applications or it can be used in color imaging applications. Besides digital cameras, image sensor 100 can also be applied in scanners, photocopy machines or facsimile machines to capture an image of an object by scanning the object sequentially.
In the case of a black-and-while application or a monochrome application, image sensor 100 is an array of photosensitive devices without the color filters. Image sensor 100 generates an image comprising an array of pixel data, each pixel data being a numerical value representing the intensity of the incident light impinged upon a pixel in the image sensor. For example, the pixel data can have a value between 0 and 255 for representing the gray scale or the variations of the light intensity of the scene being captured.
For color applications, a mosaic of selectively transmissive filters is superimposed in registration with each of the photodetectors so that a first, second, and third selective group of photodetectors are made to sense three different color ranges, for example, the red, green, and blue range of the visible spectrum, respectively. As shown in FIG. 1, image sensor 100 includes red, green and blue filters for capturing the red, green and blue components of a scene. A group of red, green and blue pixels, such as a pixel 106 for capturing the red spectrum, a pixel 107 for capturing the green spectrum, and a pixel 108 for capturing the blue spectrum, is used to make up or compose a color pixel C(i, j) in a color image. Alternately, a color imaging system may include a beam splitter and separate image sensors, each disposed to separately capture a primary color component of the target image. A color pixel C(i, j) in such a color imaging system includes the corresponding or derived pixels in each of the three primary color images.
Each color pixel C(i, j) in a color image is a vector pixel that may be expressed as follows:C(i, j)=[R(i, j) G(i, j) B(i, j)]T where (i, j) are coordinates of an image pixel in the image sensor and C refers to the color image or images captured and R, G and B represent the intensity values for each of the three color spectra. If a cluster S of corresponding pixels in the color images have an identical value, namely R(i, j)=G(i, j)<B(i, j), where (i, j) is within S, a spot in the target (a scene or an object) corresponding to the cluster S is colorless, i.e. the spot is visually somewhere from black to white. Conversely if the cluster S of the pixels in color images have different values, i.e. R(i, j)≠G(i, j)≠B(i, j), the spot in the target corresponding to the cluster S is visually colorful. For example, a pure red, green or blue vector pixel are expressed as C (i, j)=[255 0 0]T, C (i, j)=[0 255 0]T, or C (i, j)=[0 0 255]T, respectively. To ensure that a target scene or object can be exactly reproduced in a color image, the image sensor must be carefully controlled to produce color intensity values that can be combined to reproduce the colors of the target in the resultant color image.
In general, image sensors have different responses to different wavelengths of light due to the absorption properties of the underlying photodetectors. For instance, silicon sensors are more sensitive to red light than blue light. FIG. 3 illustrates response curves of an exemplary image sensor to the red, green and blue spectra. Red light has a shorter wavelength than blue light and can be more easily absorbed in most silicon sensors. The different response or sensitivities to the different light spectra in an image sensor can lead to undesirable image artifacts.
FIG. 4 illustrates the intensity curves for a red, green, and blue pixel captured by image sensor 100 at a predetermined exposure time. When the integrated light intensities of the three color pixels are captured at the same exposure time, undesirable result may occur due to the different sensitivity of the photodetectors to the different color spectra. For example, as illustrated in FIG. 4, because a silicon image sensor is more sensitive to red light than green or blue light, the red pixel may become saturated by a certain exposure time while the blue pixels have not yet integrated enough light at that same exposure time. That is, the photodetectors collecting red light may become over-exposed while those photodetectors collecting blue light may become underexposed. Previous techniques for compensating for the disparity in sensitivity levels involve stopping the light integration process when the image sensor detects that the red pixels have became saturated and then artificially saturating the blue and green color pixels. A color image thus captured may become distorted in color as the proportional ratio of each color components becomes misrepresented.
What is needed is an image sensor with improved color reproduction capability so that color images can be rendered accurately.